The rise and fall of ocean tides result from the combined gravitational pull on water by the Moon and, to a lesser extent, bu the Sun, which exerts a force on water directed towards the two astronomical bodies. These gravitational effects combine with centrifugal forces that result from the Earth and the Moon orbiting each other to make the details of tidal changes complex. This unit considers the power of the ocean tides as a potential source of useable energy and whether or not they can ever make any significant contribution to global energy supplies. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.